This invention relates to an optical subscriber network transmission system and in particular to a transmission system suitable for effecting a smooth upgrading of the transmission capacity, in the case where information of large capacity is offered to subscribers by means of single mode optical fibers.
FIG. 4 shows an example of the network offering various sorts of service by means of optical fibers. Service information is transmitted from a center 1 to each of subscribers 3 through an optical fiber 2. Facsimile, pay TV, high definition TV, etc. may be cited in addition to the conventional aural telephonic communication as the content of the service. As a system of this kind, e.g. "Integrated Circuit for Broad-Band Communication Systems" by H. Reiner, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. SAC-4, No. 4, JULY 1986, pp. 480-487 can be cited.
The service can be classified into two types, symmetric and asymmetric. These are defined, respectively as the case in which the information transmission speed in the line from the center to each of the subscribers (downstream transmission line) is equal to that in the line from each of the subscribers to the center (upstream transmission line), such as telephone and facsimile, and the case in; which the capacity of the downstream transmission line is larger than that of the upstream transmission line, such as pay TV and high definition TV.
In general, in an optical subscriber network transmission system, since the symmetric and the asymmetric services have their respective, upstream signals multiplexed onto the upstream transmission line and their respective downstream signals multiplexed onto the same downstream line, the transmission capacity of the downstream transmission line is larger than that of the upstream transmission line.
Further, since single mode fibers having an extremely wide bandwidth are used for the transmission of signals between the center and the subscribers, even the case where a subscriber desires an option for in future service having a still larger capacity of information, it is sufficient to substitute only the transmitter and receiver with ones of higher information transmission capacity, while still using the installed fibers.
In this case, it is more economical by existing equipment to divert the transmitter and receiver for the downstream transmission line to that for the upstream transmission line and to install a new transmitter and receiver having a larger capacity for the downstream transmission line than to install new transmitted and receivers for both the upstream and downstream transmission lines. This will be explained below, referring to FIGS. 5A and 5B.
FIG. 5A shows an example of transmission between the center 1 and a subscriber 3, in the case where he subscribes to the telephone and the pay TV. Aural signals from a telephone set 11 are transmitted by the transmitter 12 to the center 1 through an optical fiber 2-1 in the upstream transmission line and received by the receiver 13.
On the other hand, in the downstream transmission line, aural signals applied to a terminal 15 and pay TV signals to another terminal 17. They are multiplexed by a multiplexer 17 and transmitted by a transmitter 18 through an optical fiber 2-2 in the downstream transmission line. They are received by a receiver 19 and demultiplexed by a demultiplexer 20 into aural and pay TV signals, which are applied to the telephone set 11 and a video receiver 21, respectively.
FIG. 5B shows an example, in which facsimile and high definition TV are added in the system described above. In this case, in the upstream transmission line, apart from aural signals facsimile signals coming from a facsimile set 31 are transmitted to the center side after having been multiplexed by a multiplexer 32. They are demultiplexed by a demultiplexer 33 into the aural and facsimile signals.
Through the downstream transmission line, aural signals applied to a terminal 51 of a demultiplexer 54, pay TV multiplex signals facsimile signals applied to a terminal 52 and high definition TV signals applied to a terminal 53 are multiplexed and transmitted. They are demultiplexed by a demultiplexer 57 and applied to respective receivers.
When the system indicated in FIG. 5A is upgraded to that indicated in FIG. 5B, it is economical to divert the transmitters and receivers 18 - 19 for the downstream transmission line to those for the upstream transmission line in FIG. 5B. However it is necessary therefor to interrupt the operation of the system during a fairly long term for the upgrading work. That is, the transmitter 18 and the receiver 19 in FIG. 5A are moved and substituted for, the transmitter 12 and the receiver 13 and a transmission line is tested. Then, as indicated in FIG. 5B, the multiplexer 32 and the demultiplexer 33 are added to the transmission line and the global upstream transmission line is tested. Then the transmitter 55 and the receiver 56 are newly added to the optical fiber 2-2 in the downstream transmission line and a transmission test is effected. Thereafter, the multiplexer 54 and the demultiplexer 57 are added and the global downstream test is performed. At last, the upstream and downstream transmission lines are tested globally. In the case where this procedure is executed together for several to several tens of subscribers, service interruption of about one week may be needed.
A plurality of transmission lines are used in parallel in U.S. Pat. No. 4,658,394 by Cheng et.al. However, no service interruption described above is taken into account. Further, in "FT3C-A Lightware System for Metropolitan and Intercity Applications" by Stauffer, IEEE Journal of Selected Areas in Communications, Vol. SAC-1, No. 3, April 1983, pp. 413-419, a spare transmission line including transmitters, receivers and relays is used and no attention is paid to effect economically an upgrading of a subscriber network system.